Electronic device with wire bonds adhered between integrated circuits dies and printed circuit boards

ABSTRACT

An electronic device that has an integrated circuit die with a plurality of contacts pads, a printed circuit board with a plurality of conductors corresponding to each of the contact pads respectively, wire bonds electrically connecting each of the contact pads to the corresponding conductors and, an adhesive surface positioned between the contacts pads and the corresponding conductors. The wire bonds are secured to the adhesive surface to hold them in a low profile configuration.

The present application is a Continuation-In-Part of U.S. Ser. No. 11/860,539 (Our Docket MPN008US), filed Sep. 25, 2007, the contents of which are incorporated herein by cross reference.

FIELD OF THE INVENTION

The invention relates to the field of integrated circuits. In particular, the invention is directed to the wire bonds between a circuit board and the contact pads on the integrated circuit die.

CO-PENDING APPLICATIONS

The following application has been filed by the Applicant simultaneously with the present application:

-   -   MPN015US MPN016US MPN018US

The disclosure of this co-pending application is incorporated herein by reference. The above application has been identified by its filing docket number, which will be substituted with the corresponding application number, once assigned.

CROSS REFERENCES TO RELATED APPLICATIONS

Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/Patent applications filed by the applicant or assignee of the present invention:

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6832717 6832717 7182247 7120853 7082562 6843420 10/291718 6789731 7057608 6766944 6766945 7289103 10/291559 7299969 7264173 10/409864 7108192 10/537159 7111791 7077333 6983878 10/786631 7134598 10/893372 6929186 6994264 7017826 7014123 7134601 7150396 10/971146 7017823 7025276 7284701 7080780 11/074802 7334739 11749158 11/842948 12015477 12025746 12025747 12025748 12025749 12025750 12025751 12025754 12025756 12025757 12025759 12025760 12025761 12025762 12025764 12025765 12025766 12025767 12025768 10/492169 10/492152 10/492168 10/492161 7308148 10/502575 10/531229 10/683151 10/531733 10/683040 10/510391 10/919260 10/510392 10/778090 11/944404 11/936638 12031615 6957768 6957768 09/575172 09/575172 7170499 7170499 7106888 7106888 7123239 7123239 6982701 6982703 7227527 6786397 6947027 6975299 7139431 7048178 7118025 6839053 7015900 7010147 7133557 6914593 10/291546 6938826 7278566 7123245 6992662 7190346 11/074800 11/074782 11/074777 11/075917 7221781 11/102843 7213756 11/188016 7180507 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11/066159 11/066158 7287831 11/875936 12017818 6804030 6807315 6771811 6683996 7271936 7304771 6965691 7058219 7289681 7187807 7181063 11/338783 11/603823 11/650536 12025633 10/727181 10/727162 10/727163 10/727245 7121639 7165824 7152942 10/727157 7181572 7096137 7302592 7278034 7188282 10/727159 10/727180 10/727179 10/727192 10/727274 10/727164 10/727161 10/727198 10/727158 10/754536 10/754938 10/727227 10/727160 10/934720 7171323 7278697 11/442131 11/474278 11/488853 7328115 11749750 11749749 11955127 11951213 10/296522 6795215 7070098 7154638 6805419 6859289 6977751 6398332 6394573 6622923 6747760 6921144 10/884881 7092112 7192106 11/039866 7173739 6986560 7008033 11/148237 7222780 7270391 7150510 11/478599 11/499749 11/521388 11/738518 11/482981 11/743662 11/743661 11/743659 11/743655 11/743657 11/752900 11926109 11/927163 11929567 7195328 7182422 11/650537 11/712540 10/854521 10/854522 10/854488 7281330 10/854503 10/854504 10/854509 7188928 7093989 10/854497 10/854495 10/854498 10/854511 10/854512 10/854525 10/854526 10/854516 7252353 10/854515 7267417 10/854505 10/854493 7275805 7314261 10/854490 7281777 7290852 10/854528 10/854523 10/854527 10/854524 10/854520 10/854514 10/854519 10/854513 10/854499 10/854501 7266661 7243193 10/854518 10/854517 10/934628 7163345 7322666 11/601757 11/706295 11/735881 11748483 11749123 11/766061 11775135 11772235 11/778569 11/829942 11/870342 11/935274 11/937239 11961907 11961940 11961961 11/014731 D529081 D541848 D528597 6924907 6712452 6416160 6238043 6958826 6812972 6553459 6967741 6956669 6903766 6804026 7259889 6975429 10/636234 10/636233 7301567 10/636216 7274485 7139084 7173735 7068394 7286182 7086644 7250977 7146281 7023567 7136183 7083254 6796651 7061643 7057758 6894810 6995871 7085010 7092126 7123382 7061650 10/853143 6986573 6974212 7307756 7173737 10/954168 7246868 11/065357 7137699 11/107798 7148994 7077497 11/176372 7248376 11/225158 7306321 7173729 11/442132 11/478607 11/503085 11/545502 11/583943 11/585946 11/653239 11/653238 11/764781 11/764782 11/779884 11845666 11/872637 11/944401 11/940215 11/544764 11/544765 11/544772 11/544773 11/544774 11/544775 11/544776 11/544766 11/544767 11/544771 11/544770 11/544769 11/544777 11/544768 11/544763 11/293804 11/293840 11/293803 11/293833 11/293834 11/293835 11/293836 11/293837 11/293792 11/293794 11/293839 11/293826 11/293829 11/293830 11/293827 11/293828 7270494 11/293823 11/293824 11/293831 11/293815 11/293819 11/293818 11/293817 11/293816 11/838875 11/482978 11/640356 11/640357 11/640358 11/640359 11/640360 11/640355 11/679786 11/872714 10/760254 10/760210 10/760202 7201468 10/760198 10/760249 7234802 7303255 7287846 7156511 10/760264 7258432 7097291 10/760222 10/760248 7083273 10/760192 10/760203 10/760204 10/760205 10/760206 10/760267 10/760270 7198352 10/760271 7303251 7201470 7121655 7293861 7232208 7328985 10/760261 7083272 7261400 11/474272 11/474315 7311387 11/583874 7303258 11/706322 11/706968 11/749119 11749157 11779848 11/782590 11/855152 11855151 11/870327 11/934780 11/935992 11951193 12/017327 12015273 12036882 11/014764 11/014763 7331663 11/014747 7328973 11/014760 11/014757 7303252 7249822 11/014762 7311382 11/014723 11/014756 11/014736 11/014759 11/014758 11/014725 7331660 11/014738 11/014737 7322684 7322685 7311381 7270405 7303268 11/014735 11/014734 11/014719 11/014750 11/014749 7249833 11/758640 11/775143 11/838877 11944453 11/944633 11955065 12/003952 12007818 12007817 12071187 11/014769 11/014729 7331661 11/014733 7300140 11/014755 11/014765 11/014766 11/014740 7284816 7284845 7255430 11/014744 7328984 11/014768 7322671 11/014718 11/014717 11/014716 11/014732 11/014742 11/097268 11/097185 11/097184 11/778567 11852958 11852907 11/872038 11955093 11961578 12022023 12023000 12023018 12031582 12043708 11/293820 11/293813 11/293822 11/293812 11/293821 11/293814 11/293793 11/293842 11/293811 11/293807 11/293806 11/293805 11/293810 11/688863 11/688864 11/688865 11/688866 11/688867 11/688868 11/688869 11/688871 11/688872 11/688873 11/741766 12014767 12014768 12014769 12014770 12014771 12014772 12014773 12014774 12014775 12014776 12014777 12014778 12014779 12014780 12014781 12014782 12014783 12014784 12014785 12014787 12014788 12014789 12014790 12014791 12014792 12014793 12014794 12014796 12014798 12014801 12014803 12014804 12014805 12014806 12014807 61034147 11/482982 11/482983 11/482984 11/495818 11/495819 11/677049 11/677050 11/677051 11872719 11872718 61033357 7306320 11/934781 D528156 10/760180 7111935 10/760213 10/760219 10/760237 7261482 10/760220 7002664 10/760252 10/760265 7088420 11/446233 11/503083 11/503081 11/516487 11/599312 6364451 6533390 6454378 7224478 6559969 6896362 7057760 6982799 11/202107 11/743672 11744126 11/743673 7093494 7143652 7089797 7159467 7234357 7124643 7121145 7089790 7194901 6968744 7089798 7240560 7137302 11/442177 7171855 7260995 7260993 7165460 7222538 7258019 11/543047 7258020 11/604324 7334480 11/706305 11/707056 11744211 11/767526 11/779846 11/764227 11/829943 11/829944 12015390 12031475 6454482 6808330 6527365 6474773 6550997 7093923 6957923 7131724 10/949288 7168867 7125098 11/706966 11/185722 7249901 7188930 11/014728 11/014727 D536031 D531214 7237888 7168654 7201272 6991098 7217051 6944970 10/760215 7108434 10/760257 7210407 7186042 10/760266 6920704 7217049 10/760214 10/760260 7147102 7287828 7249838 10/760241 10/962413 10/962427 7261477 7225739 10/962402 10/962425 10/962428 7191978 10/962426 10/962409 10/962417 10/962403 7163287 7258415 7322677 7258424 10/962410 7195412 7207670 7270401 7220072 11/474267 11/544547 11/585925 11/593000 11/706298 11/706296 11/706327 11/730760 11/730407 11/730787 11/735977 11/736527 11/753566 11/754359 11/778061 11/765398 11/778556 11/829937 11/780470 11/866399 11/223262 11/223018 11/223114 11955366 7322761 11/223021 11/223020 11/223019 11/014730 D541849 29/279123 6716666 6949217 6750083 7014451 6777259 6923524 6557978 6991207 6766998 6967354 6759723 6870259 10/853270 6925875 10/898214 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7334873 6582059 10/510152 6513908 7246883 6540332 6547368 7070256 6508546 10/510151 6679584 7303254 6857724 10/509998 6652052 10/509999 6672706 10/510096 6688719 6712924 6588886 7077508 7207654 6935724 6927786 6988787 6899415 6672708 6644767 6874866 6830316 6994420 6954254 7086720 7240992 7267424 7128397 7084951 7156496 7066578 7101023 11/165027 11/202235 11/225157 7159965 7255424 11/349519 7137686 7201472 7287829 11/504602 7216957 11/520572 11/583858 11/583895 11/585976 11/635488 7278712 11/706952 11/706307 7287827 11944451 11/740287 11/754367 11/758643 11/778572 11859791 11/863260 11/874178 11/936064 11951983 12015483 6916082 6786570 10/753478 6848780 6966633 7179395 6969153 6979075 7132056 6832828 6860590 6905620 6786574 6824252 7097282 6997545 6971734 6918652 6978990 6863105 10/780624 7194629 10/791792 6890059 6988785 6830315 7246881 7125102 7028474 7066575 6986202 7044584 7210762 7032992 7140720 7207656 7285170 11/048748 7008041 7011390 7048868 7014785 7131717 7284826 7331101 7182436 7104631 7240993 7290859 11/202217 7172265 7284837 7066573 11/298635 7152949 7334877 11/442133 7326357 7156492 11/478588 7331653 7287834 11/525861 11/583939 11/545504 7284326 11/635485 11/730391 11/730788 11/749148 11/749149 11/749152 11/749151 11/759886 11/865668 11/874168 11/874203 11971182 12021086 12015441 11965722 6824257 7270475 6971811 6878564 6921145 6890052 7021747 6929345 6811242 6916087 6905195 6899416 6883906 6955428 7284834 6932459 6962410 7033008 6962409 7013641 7204580 7032997 6998278 7004563 6910755 6969142 6938994 7188935 10/959049 7134740 6997537 7004567 6916091 7077588 6918707 6923583 6953295 6921221 7001008 7168167 7210759 7337532 7331659 7322680 6988790 7192120 7168789 7004577 7052120 11/123007 6994426 7258418 7014298 7328977 11/177394 7152955 7097292 7207657 7152944 7147303 7338147 7134608 7264333 7093921 7077590 7147297 20060038853 11/248832 11/248428 11/248434 7077507 7172672 7175776 7086717 7101020 11/329155 7201466 11/330057 7152967 7182431 7210666 7252367 7287837 11/485255 11/525860 6945630 7018294 6910014 6659447 6648321 7082980 6672584 7073551 6830395 7289727 7001011 6880922 6886915 6644787 6641255 7066580 6652082 7284833 6666544 6666543 6669332 6984023 6733104 6644793 6723575 6953235 6663225 7076872 7059706 7185971 7090335 6854827 6793974 10/636258 7222929 6739701 7073881 7155823 7219427 7008503 6783216 6883890 6857726 10/636274 6641256 6808253 6827428 6802587 6997534 6959982 6959981 6886917 6969473 6827425 7007859 6802594 6792754 6860107 6786043 6863378 7052114 7001007 10/729151 10/729157 6948794 6805435 6733116 10/683006 7008046 6880918 7066574 6983595 6923527 7275800 7163276 7156495 6976751 6994430 7014296 7059704 7160743 7175775 7287839 7097283 7140722 11/123009 11/123008 7080893 7093920 7270492 7128093 7052113 7055934 11/155627 7278796 11/159197 7083263 7145592 7025436 11/281444 7258421 11/478591 7332051 7226147 11/482940 7195339 11/503061 11/505938 7284838 7293856 11/544577 11/540576 7325901 11/592991 11/599342 11/600803 11/604321 11/604302 11/635535 11/635486 11/643842 11/655987 11/650541 11/706301 11/707039 11/730388 11/730786 11/730785 11/739080 7322679 11/768875 11/779847 11/829940 11847240 11/834625 11/863210 11/865680 11/874156 11/923602 11951940 11954988 11961662 12015178 12015157 12/017305 12017926 12015261 12025605 12031646 7067067 6776476 6880914 7086709 6783217 7147791 6929352 7144095 6820974 6918647 6984016 7192125 6824251 6834939 6840600 6786573 7144519 6799835 6959975 6959974 7021740 6935718 6938983 6938991 7226145 7140719 6988788 7022250 6929350 7011393 7004566 7175097 6948799 7143944 7310157 7029100 6957811 7073724 7055933 7077490 7055940 10/991402 7234645 7032999 7066576 7229150 7086728 7246879 7284825 7140718 7284817 7144098 7044577 7284824 7284827 7189334 7055935 7152860 11/203188 11/203173 7334868 7213989 11/225156 11/225173 7300141 7114868 7168796 7159967 7328966 7152805 11/298530 11/330061 7133799 11/330054 11/329284 7152956 7128399 7147305 7287702 7325904 7246884 7152960 11/442125 11/454901 11/442134 11/450441 11/474274 11/499741 7270399 6857728 6857729 6857730 6989292 7126216 6977189 6982189 7173332 7026176 6979599 6812062 6886751 10/804057 10/804036 7001793 6866369 6946743 7322675 6886918 7059720 7306305 10/846562 7334855 10/846649 10/846627 6951390 6981765 6789881 6802592 7029097 6799836 7048352 7182267 7025279 6857571 6817539 6830198 6992791 7038809 6980323 7148992 7139091 6947173 7101034 6969144 6942319 6827427 6984021 6984022 6869167 6918542 7007852 6899420 6918665 6997625 6988840 6984080 6845978 6848687 6840512 6863365 7204582 6921150 7128396 6913347 7008819 6935736 6991317 7284836 7055947 7093928 7100834 7270396 7187086 7290856 7032825 7086721 7159968 7010456 7147307 7111925 11/144812 7229154 11/505849 11/520570 7328994 7341672 11/540575 11/583937 7278711 7290720 7314266 11/635489 11/604319 11/635490 11/635525 7287706 11/706366 11/706310 11/706308 11/785108 11/744214 11744218 11748485 11/748490 11/764778 11/766025 11/834635 11839541 11860420 11/865693 11/863118 11/866307 11/866340 11/869684 11/869722 11/869694 11/876592 11/945244 11951121 11/945238 11955358 11965710 11962050 12015478 12015423 12015434 12023015 12030755 12025641 12036279 12031598

BACKGROUND OF THE INVENTION

An integrated circuit fabricated on the silicon wafer is often referred to as a ‘die’. For the purposes of this specification, the term die will be used as a reference to an integrated circuit fabricated on a wafer substrate using lithographic the well known etching and deposition techniques commonly used in semiconductor fabrication. Integrated circuit (IC) dies are electrically connected to printed circuit boards by wire bonds. The wire bonds are very thin wires—around 25 to 40 microns in diameter—extending from contact pads along the side of the wafer substrate to contacts on the printed circuit board (PCB). Wire bonding is a widely used electrical interconnection technique because of the speed and accuracy of modern wire bonding machines, commonly referred to as wirebonders.

Wirebonders are automated devices that weld small lengths of wire from conductors on the PCB to the contact pads on an integrated circuit die. Wire is fed through a bonding tool that uses some combination of pressure, heat and/or ultra-sonic energy to attach the wire to the bond pads via a solid phase welding process. The two most common types of wire bonder are referred to as wedge bond and ball bond. These refer to the bonding tool and the configuration of the wire bond itself. With both types of wirebonders, the individual wire bonds extend in an arc from the bond pad on the integrated circuit (IC) die to the conductor on the PCB. This is because wires from the contact pads to the PCB are made longer than necessary to accommodate changes in the gap between the PCB and the bonds pads due to thermal expansion, flex in the components and so on.

To protect and strengthen the wire bonds, they are sealed within a bead of epoxy called encapsulant. The top of the wire arc is often about 300 microns above the contact pads although some wire bonding may extend even higher. As the name suggests, the encapsulant needs to encapsulate the full length of the wire so the encapsulant bead will extend 500 microns to 600 microns proud of the contact pads.

If the die is purely an electronic microprocessor, there is little need to keep close control of the encapsulant bead dimensions. However, if the die is a micro-electro mechanical systems (MEMS) device with an active upper surface, it may be necessary or desirable to bring the active surface of the die onto close proximity with another surface. One such situation applies to inkjet printheads. The proximity of the print media to the nozzle array influences the print quality. Similarly, if a cleaning surface is wiped across the nozzles, the bead of encapsulant can hamper the wiping contact.

Another problems arises because of sides of the encapsulant bead are not straight. One commonly used technique for depositing the encapsulant involves extruding it from a needle directly onto the line of wire bonds. The encapsulant volume and placement on the die is not very accurate. Variations in the pressure from the pump or slight non-uniformities in the speed of the needle cause the side of the bead contacting the active surface to be reasonably crooked. As the side of the bead is not straight, it has to be generously spaced from any active parts on the active surface to comfortably accommodate the perturbations. Spacing the electrical contacts away from the active portions (say for example, inkjet nozzles) of the active surface uses up valuable wafer real estate and reduces the number of dies that can be fabricated from a wafer disc.

In light of the widespread use of inkjet printheads, the invention will be described with specific reference to its application in this field. However, the ordinary worker will appreciate that this is purely illustrative and the invention is equally applicable to other integrated circuits wire bonded to a PCB or other support structure.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method of profiling a series of wire bonds between a line of contact pads on a die, and a corresponding set of conductors on a supporting structure, the method comprising the steps of:

electrically connecting each of the contact pads on the die to a corresponding conductor on the supporting structure with a respective wire bond, each of the wire bonds extending in an arc from the contact pad to the conductor; and,

pushing on each of the wire bonds individually to collapse the arc and plastically deform the wire bond such that the plastic deformation maintains the wire bond in a flatter profile shape.

The strength of the wire bond is known to be relatively small; of the order of 3 to 5 grams force. However, the Applicant's work has found that the wire bond structure is robust enough to withstand a certain degree of work hardening from plastic deformation. The arc of the wire bond can be deformed into a flatter profile without compromising the electrical connection with the PCB. The Applicant's above referenced U.S. Ser. No. 11/860,539 (Our Docket MPN008US) discloses a technique for simultaneously pushing some or all of the wire loops in the line of wire bonds. This so called gang wire pushing technique is effective but further development has shown that individually collapsing each wire bond is more controlled and easier to implement in a high volume manufacturing process.

Preferably, adjacent wire bonds in the line of wire bonds are sequentially pushed. In a further preferred form, the step of forming the line of wire bonds uses a wirebonder that has a bonding tool for moving between the contacts pads and their respective corresponding conductors, and the line of wires bonds are sequentially pushed by a wire engaging structure on the wirebonder. Preferably, the wire engaging structure and the bonding tools are configured for synchronized movement. Preferably, the wire engaging structure pushes the wire bond immediately adjacent the wire bond currently being formed by the bonding tool. Preferably, the wirebonder is a wedge type wirebonder and the bonding tool is a wedge with a wire clamp at a distal end such that during use, the wire clamp holds a piece of wire in contact with one of the contact pads on the die to form a weld connection before moving to the corresponding conductor on the PCB to weld the other end of the wire and formal wire bond, and the wire engaging structure has a wire pushing surface that contacts the wire bond, the wire pushing surface is adjacent to, and 1.0 mm to 1.6 mm behind the wire clamp with respect to the movement of the wedge towards the IC die. In a particularly preferred form, the wire pushing surface is between 50 microns to 400 microns closer to the PCB and the wire clamp. In some embodiments, the line of wire bonds does not extend more than 150 microns above the contact pads of the IC die. In preferred embodiments, the line of wire bonds does not extend more than 50 microns above the contact pads of the IC die. In a particularly preferred form, the wire bonds are attached to the contact pads with a bond strength greater than 3 g force.

Preferably the contact pads are spaced from the corresponding conductors on the PCB by more than 1 mm. In a further preferred form, the contact pads are between 2 mm and 3 mm from the corresponding conductors on the PCB. In some embodiments, the PCB has a support structure and the flex PCB and need to the support structure such that the conductors are adjacent the contact pads on the die. In particular embodiments, the support structure has a chip mounting area for supporting the die, the die having a back surface in contact with the chip mounting area and an active surface opposing the back surface, the active surface having the contact pads, and the chip mounting area being raised relative to the remainder of the support structure such that the contact pads are raised relative to the conductors. In a particularly preferred form, the support structure is a liquid crystal polymer (LCP) molding. Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a particularly preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. In some embodiments, the support structure is a liquid crystal polymer (LCP) molding.

Preferably, the wire bonds are covered in a bead of encapsulant, the bead of encapsulant extending less than 200 microns above the active surface of the die.

Preferably, the wire bonds are covered in a bead of encapsulant, the bead of encapsulant having a profiled surface that is flat, parallel to and spaced less than 100 microns from the active surface.

Preferably, the line of wire bonds are covered in a bead of encapsulant, the bead of encapsulant having a profiled surface that is flat and inclined relative to the active surface.

Preferably, the wire bonds are covered in a bead of encapsulant, the encapsulant being an epoxy material that is thixotropic when uncured.

Preferably, the wire bonds are covered in a bead of encapsulant, the encapsulant being an epoxy material has a viscosity greater than 700 cp when uncured.

In a particular embodiment, the printhead IC is mounted in a printer such that during use the nozzles are less than 100 microns from the paper path.

According to a second aspect, the present invention provides a wirebonder for electrically connecting an integrated circuit die with conductors on a printed circuit board, the wirebonder comprising:

a bonding tool for attaching wire bonds from the integrated circuit die to the conductors of the printed circuit board; and,

a wire engaging structure for deforming the wire bonds.

Preferably, the wire engaging structure pushes on the wire bonds to plastically deform them.

Preferably, the wire engaging structure is configured to push the wire bonds onto an adhesive surface positioned between the integrated circuit and the conductors of the printed circuit board.

Preferably, the wire engaging structure is flexible relative to the bonding tool.

Preferably, the wire engaging structure is configured for synchronized movement with the bonding tool.

Preferably, the bonding tool moves from the integrated circuit to the conductors when forming one of the wire bonds and the wire engaging structure has a wire pushing surface positioned 1.0 mm to 1.6 mm behind the bonding tool with respect to its direction of movement when forming the wire bonds.

Preferably, the integrated circuit die is mounted to a supporting surface and the wire pushing surface is 50 microns to 400 microns closer to the supporting surface than the bonding tool. Preferably, the wirebonder is a wedge type wirebonder and the bonding tool is a wedge with a wire clamp at a distal end such that during use, the wire clamp holds a piece of wire in contact with one of the contact pads on the die to form a weld connection before moving to the corresponding conductor on the PCB to weld the other end of the wire and formal wire bond. Preferably, the wire pushing surface is formed from a material that has a hardness less than that of the wire bonds.

Preferably, the wire bonds are formed from lengths of wire with a gauge between 15 microns and 75 microns. In a particularly preferred form, the gauge is about 25 microns.

In some embodiments, the wire bonds attach to respective contact pads on the IC die and the wire bonds do not extend more than 150 microns above the contact pads of the IC die. In preferred embodiments, the line of wire bonds does not extend more than 50 microns above the contact pads of the IC die.

Preferably the contact pads are spaced from the corresponding conductors on the PCB by more than 1 mm. In a further preferred form, the contact pads are between 2 mm and 3 mm from the corresponding conductors on the PCB. In some embodiments, the PCB has a support structure and the flex PCB and need to the support structure such that the conductors are adjacent the contact pads on the die. In particular embodiments, the support structure has a chip mounting area for supporting the die, the die having a back surface in contact with the chip mounting area and an active surface opposing the back surface, the active surface having the contact pads, and the chip mounting area being raised relative to the remainder of the support structure such that the contact pads are raised relative to the conductors. In a particularly preferred form, the support structure is a liquid crystal polymer (LCP) molding. Preferably, the active surface has functional elements spaced less than 260 microns from the contacts pads of the die. In a particularly preferred form, the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected. In some embodiments, the support structure is a liquid crystal polymer (LCP) molding.

According to a third aspect, the present invention provides an electronic device comprising:

an integrated circuit die with a plurality of contacts pads;

a printed circuit board with a plurality of conductors corresponding to each of the contact pads respectively;

wire bonds electrically connecting each of the contact pads to the corresponding conductors; and,

an adhesive surface positioned between the contacts pads and the corresponding conductors; wherein,

the wire bonds are secured to the adhesive surface.

According to a fourth aspect, the present invention provides a method of reducing wire bond loop heights in wire bonds electrically connecting an integrated circuit die with a contact pad to a printed circuit board with a conductor, the method comprising the steps of:

mounting the integrated circuit die such that the contact pad is spaced from the conductor;

positioning an adhesive surface between the contact pad and the conductor on the printed circuit board;

attaching wire to one of the contact pad or the conductor;

drawing the wire towards the other of the contact pad or the conductor;

allowing the wire to contact the adhesive surface; and,

attaching the wire to the other of the contact pad of the conductor to form a wire bond adhered to the adhesive surface and a point intermediate its ends.

These aspects of the invention are placed on the realisation that wire bonds can be adhered to an underlying supporting structure without detrimental effects to their bond strength or function. Adhering the wire bonds between their ends provides a reliable and control production in the wire bond loop height. The resulting wire bond heights can be smaller than that achieved by inducing plastic deformation as the individual wire is unable to spring back up when the wire engaging structure disengages. It will be appreciated that plastically deforming a wire bond also involves initially elastically deforming the wire. The elastic deformation is removed when the wire pushing structure is retracted.

The wire bond can be adhered while the bonds are being formed by the wirebonder without any modification to the bonding tool. The applicant has found that wire bond is will typically allow the wire to touch the surface between the die and the conductors on the printed circuit board as the bond is being formed. Once the wire has been welded to the contact pad on the die, the bonding tool draws is towards the conductors on the printed circuit board. As it is drawn across the gap between the die and printed circuit board, the wire drapes downwardly and rests on the underlying surface. The only once the bonding tool has welded the other end of the wire to the conductor, and the wire clamp immediately behind the bonding tool breaks off the feed wire by pulling until tensile failure, does residual tension in the loop cause it to bow upwards. If the wire is brought down to touch an adhesive surface before the ultrasonic weld on the printed circuit board, it is not able to bow upwards.

Preferably, the wire bond is moved into contact with the adhesive surface by a wirebonder is the wire bond is being formed.

Preferably, the adhesive surface is one side of a double-sided adhesive tape. Preferably, the integrated circuit die and the PCB are mounted to a supporting structure such that they are adjacent and spaced from each other. Preferably, the PCB is a flexible PCB and the supporting structure is a liquid crystal polymer (LCP) molding. Preferably, the integrated circuit die is mounted to the supporting structure by a die attach film, and the adhesive surface is provided by a portion of the die attach film.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a common prior art technique for applying a bead of encapsulant to wire bonds;

FIG. 2 is a schematic representation of a die mounted to a supporting structure with a chip mounting area raised relative to the flex PCB mounting area;

FIGS. 3A, 3B and 3C are schematic representations of the encapsulant bead being profiled into a desired shape using a moveable blade;

FIGS. 4A to 4D are schematic representations of wire bonds being profiled by plastic deformation;

FIGS. 5A and 5B show the encapsulant bead height reductions for plastically deformed wire bonds;

FIGS. 6A to 6C show the encapsulant bead being applied to the wire bonds using the profiling blade;

FIGS. 7A and 7B show the profiling blade being used to control the encapsulant bead front on the surface of the die;

FIG. 8 shows the wire pusher mounted to the wirebonder;

FIG. 9 shows the wire pusher and the wire bonder in profile; and,

FIG. 10 shows the wire bonds adhered to the LCP molding between the die and the flex PCB.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a common technique used for applying a bead encapsulant to wire bonds. A die 4 is mounted to a supporting structure 6 adjacent the edge of a flex PCB 8 (flexible printed circuit board). The die 4 has a line of contact pads 10 along one edge and the flex PCB 8 has corresponding bond pads 12. Wire bonds 16 extend from the bond pads 10 to the bonds pads 12. Power and data is transmitted to the die 4 via conductive traces 14 in the flex PCB 8. This is a simplified representation of the dies mounted within many electronic devices. The printhead IC dies mounted to the LCP (liquid crystal polymer) molding to receive print data from an adjacent flex PCB, as described in U.S. Ser. No. 11/014769 (Our Docket RRC001US) incorporated herein by cross reference, is one example of this type of die mounting arrangement. The ordinary worker will appreciate that the die may also be mounted directly to a hard PCB with traces formed thereon.

The wire bonds 16 are covered in a bead on encapsulant 2 to protect and reinforce the bonds. The encapsulant 2 is dispensed from a discharge needle 18 directly onto the wire bonds 16. Often the encapsulant bead 2 is three separate beads—two beads of so-called ‘dam’ encapsulant 20, and one bead of ‘fill’ encapsulant 22. The dam encapsulant 20 has a higher viscosity than the fill encapsulant 22, and serves to form a channel to hold the fill encapsulant bead. The height H of the bead 2 above the die 4 is usually about 500-600 microns. In most electronic devices, this does not pose a problem. However, if the die has an active surface that needs to operate in close proximity to another surface, this bead can be an obstruction.

Elevating the Die Relative to the Flex PCB

FIG. 2 shows a stepped support structure 6 that has raised the chip mounting area 26 relative to the PCB mounting area 24 (or at least the area mounting the PCB bonds pads 12). With the die 4 on a raised chip mounting area 26, the arc of the wire bonds 16 are lower relative to active surface 28 of the die 4. In fact, the end of the wire bond 16 attached to the contact pad 10 can be the apex of the arc (bearing in mind that the wire bond arc is intended to accommodate some relative movement of the die and PCB). When the wire bonds 16 are covered with encapsulant 2, the bead has a reduced height H above the active surface 28 of the die 4. If the bead of encapsulant 2 uses two beads of dam encapsulant 24 and a fill encapsulant 22, the positions, volumes and viscosities of the beads need to take the step into account. Bead heights less than 100 microns are easily achievable, and with additional measures, such as wire arc collapsing and bead profiling (discussed below), bead height of less than 50 microns are possible.

With the die 4 raised above the flex PCB 8 by 410 microns, the height of the wire bonds 16 above the die is about 34 microns. With the die raised 610 microns above the flex PCB, the wire bond height is around 20 microns. Raising the die even further has shown little or no further reduction in wire bond height with a step of 710 microns having a wire bond height of around 20 microns.

Shaping the Encapsulant Bead with a Profiling Blade

FIGS. 3A to 3C show the encapsulant 2 being profiled with a profiling blade 30. The support structure 6 is again stepped to reduce the height of the wire bonds 16 above the die 4. Before the epoxy encapsulant 2 has cured, the profiling blade 30 moves across the die 4 and wire bonds in a predetermined path. As shown in FIG. 3B, the blade 30 displaces the top of the bead 30 to its flex PCB side to form a flat top surface 32 that is at a significantly reduced height H above the die 4.

The encapsulant bead 2 may be a plurality of separate beads as shown in FIGS. 1 and 2, or a single bead of one material. However, for close dimensional control of the profiled encapsulant, the encapsulant materials used should be thixotropic—that is, once deposited from the discharge needle, or profiled by the blade 30, the material should not flow under its own weight, but rather hold its form until it cures. This requires the epoxy to have an uncured viscosity greater than about 700 cp. A suitable encapsulant is DYMAX 9001-E-v3.1 Chip Encapsulant produced by Dymax Corporation with a viscosity of approximately 800 cp when uncured. The blade 30 may be ceramic (glass) or metal and preferably about 200 microns thick.

It will be appreciated that the relative movement of the blade 30 and the die 4 can be precisely controlled. This allows the height H to be determined by the tolerance of the wire bonding process. As long as H is greater than the nominal height of the wire bond arc above the die, plus the maximum tolerance, the encapsulant 2 will cover and protect the wire bonds 16. With this technique, the height H can be easily reduced from 500-600 microns to less than 300 microns. If the heights of the wire bond arcs are also reduced, the height H of the encapsulant bead can be less than 100 microns. The Applicant uses this technique to profile encapsulant on printhead dies down to a height of 50 microns at its lowest point. As shown in FIG. 3C, the lowest point is at the encapsulant front and the blade 30 forms an inclined face 32 in the top of the bead 2. The inclined face is utilized by the printhead maintenance system when cleaning the paper dust and dried ink from the nozzle face. This illustrates the technique's ability to not just reduce the height of the encapsulant bead, but to form a surface that can perform functions other than just encapsulate the wire bonds. The edge profile of the blade and the path of the blade relative to the die can be configured to form a surface that has a multitude of shapes for a variety of purposes.

Plastic Deformation of the Wire Bond Arcs

FIGS. 4A to 4C show another technique for lowering the profile of wire bonds. FIG. 4A shows the die 4 connected to the flex PCB 8 via the wire bonds 16. While the stepped support structure 6 has lowered the height of the wire bond arcs compared to a flat supporting structure, the wire bonds still have a natural tendency to bow upwards rather than downwards towards the corner of the step. The wires 16 are typically about 32 microns in diameter and have a pull force of about 3 to 5 grams force. The pull force is the tensile load necessary to break the connection to the contact pad 10 or the bond pad 12. Given the fragility of these structures (one of the reasons encapsulant is applied), conventional wisdom is to avoid any contact between the wire bond arcs and other solid surfaces.

As shown in FIG. 4B, the arc of the wire bonds 16 can be collapsed by a wire pusher 34. The wire pusher 34 displaces the wire bond 16 enough to elastically and plastically deform the arc. The Applicants have shown that contact with the wire pusher 34 can cause localized work hardening in the wire, but as long as the pushing force is not excessive, it does not break. The end of the wire pusher 34 is rounded to avoid stress concentration points. The wire pusher may be a stylus for engaging single wire bonds or a blade that pushes on multiple wire bonds simultaneously.

Referring now to FIG. 4C, the wire pusher 34 is retracted and the wire springs back toward its original shape to relieve the elastic deformation. However, the plastic deformation remains and the wire bond height above the die 4 is much reduced. Testing has shown that an initial wire bond loop height of 200 microns can be reduced to about 35 microns using this technique. Tests have also shown that the pull strength of the plastically deformed wires remains at about 3 to 5 grams force.

The collapse of the wire bonds is uncontrolled and leaves the wire bonds somewhat randomly deformed. However, pushing the wire bonds closer to the die provides more uniformly shaped collapsed wire bonds. The Applicant's work has shown that engaging the wires about 200 to 300 microns for the die provides the best results.

As shown in FIG. 4D, the die 4 and the flex PCB 8 are mounted to a flat support structure 6. As discussed above, this means the original loop height of the wire bond arc is much higher—approximately 400 microns above the die 4. Consequently, the wire has more plastic deformation when the loop is collapsed by the wire pusher. Even so, the Applicants results show that the residual loop height after pushing is about 20-50 microns.

FIGS. 5A and 5B show the collapsed wire bonds 16 covered with an encapsulant bead 2. Even without bead profiling prior to curing, the height H of the bead above the die is much less than the bead necessary to encapsulate the original undeformed wire loops.

Applying Encapsulant with Profiling Blade

FIGS. 6A, 6B and 6C show the application of the encapsulant bead using the profiling blade 30 instead of a discharge needle (see FIGS. 1 and 2). As previously discussed, the flowrate of encapsulant from the discharge needle can vary and this gives rise to large variations on the position of the encapsulant front on the active surface of the die 4. Consequently, any functional elements in the active surface of the die need to be sufficiently spaced from the contacts pads 10 to allow for the meandering encapsulant front.

Applying the encapsulant with the profiling blade avoids the problems caused by the flowrate fluctuations from the discharge needle. As shown in FIG. 6A, the bead of encapsulant 40 can be formed on the profiling blade 30 by simply dipping it into a reservoir of uncured encapsulant epoxy. Of course, the bead 40 may also be formed by any other convenient method, such as running the discharge needle along one end of the blade 30.

FIG. 6B show the blade 30 having been lowered to touch the bead 40 onto the die 4. When the encapsulant material touches the die surface, it wets and wicks along the surface while remaining pinned to the edge of the blade. The blade 30 is held at a predetermined height above the die 4 and moved over the bead 2 to flatten and lower its profile. The encapsulant displaced from the top of the bead 2 by the blade 30, spreads over the PCB side of the bead 2. It is not relevant if the encapsulant spreads further over the PCB than necessary. As long as the wire bonds 16 and the bonds pads 12 are covered, any additional encapsulant on the PCB 8 surface is not detrimental.

In FIG. 6C, the wire bond 16 height has been reduced by collapsing the arc in accordance with the techniques discussed above. As previously discussed, the bead 2 deposited by the discharge needle need not be as big to cover the wire bond 16 once it has been collapsed. Furthermore, the blade 30 can be brought closer to the die 4 without contacting wire bonds 16 when profiling the encapsulant 2. Hence the bead profile in FIG. 6C is substantially lower than that of FIG. 6B.

Encapsulant Front Control

When the encapsulant material is dispensed from the discharge needle, minor variations in the flowrate can cause the bead to bulge at points of higher flow. Consequently, the side of the bead that contacts the active surface of the die is not straight, but has significant perturbations. These perturbations have to be accommodated between the contact pads and any functional elements on the active surface. The spacing between the contacts pads and the functional elements consumes valuable ‘chip real estate’. The Applicant has previously developed printhead dies with a spacing of 260 microns between the contact pads and the first row of nozzles. Better control of the encapsulant front reduces the space between the contacts and operational elements, and so the overall dimensions of the die. Hence the design can be more compact and more chips fabricated from the original wafer disc.

As shown in FIGS. 7A and 7B, the profiling blade 30 is used to control the front 36 of the bead of encapsulant 2. The blade 30 is positioned over the die 4 to define a gap 42 between its lower edge and the active surface 28. As the discharge needle 18 dispenses the encapsulant material 44, it flows onto the active surface, one side of the blade and a fillet of the material extends through the gap 42. Because of the flow restriction created by the gap, flow variations have a reduced effect on the dimensions of the fillet that flows through the gap. Therefore the encapsulant front 36 closely corresponds to the line of the lower edge of the blade 30.

As shown in FIG. 7B, the profiling blade 30 is already in position to profile the encapsulant bead 2 once it has been dispensed from the discharge needle. The blade 30 simply moves over the die 4 in a direction away from the nozzles 38. This keeps the encapsulant front 36 in place and flattens the profile of the encapsulant bead 2 over the wire bonds 16.

Collapsing the Wire Bonds Arcs with the Wirebonder

FIGS. 8 and 9 show a technique for individually deforming each of the wire bond arcs using the wirebonder. This has several advantages over the technique shown in FIGS. 4A to 4C described in the “PLASTIC DEFORMATION OF THE WIRE BOND ARCS” section above. Firstly, deforming the wire bonds as they are being attached by the wirebonder is more time efficient than pushing the line of wire bonds as a separate production step. Secondly, it has been found that individually engaging and deforming each wire in the line of wire bonds provides uniform results in terms of wire deformation and bond strength.

FIG. 8 is a schematic perspective of a wirebonder 46 forming the individual wire bonds 16 between the contact pads 10 and the conductors 12 on the flex PCB 8. The printhead IC 4 is shown adhered to one side of the die attach film 58. In turn, the die attach film 58 is attached to the LCP molding 6. Laser ablated holes through the die attach film 58 feed ink to the array of nozzles 38. The LCP molding six has a stepped surface 60 so that the printhead IC 4 is raised relative to the flex PCB eight. As discussed above this helps to reduce the height of the wire bonds 16.

The wire bonder 46 is commonly known in the industry as a “wedge type” wire bonder. The wedge 48 receives a stock of feed wire 56 at its tip. Using some combination of pressure, heat and ultrasonic energy, the end of the wire bond 16 is welded to one of the conductors 12 on the flex PCB 8, or one of the contact pads 10 on the printhead integrated circuit 4. FIG. 8 includes an enlarged inset showing the connection between the wire bond 16 and the conductor 12. The end of the wire has a flatten section 54 created by the tip of wedge 48. Adjacent to flatten section 54 is a neck portion 52 where the wire 16 transitions to its circular cross section. This neck section of the wire is work hardened and particularly prone to plastic deformation. In light of the greater propensity for plastic deformation at the neck portion 52, the wire bonder 46 is fitted with a wire engaging structure 50 pushes on the wire bond 16 in this region. However, the wire engaging structure 50 to not contact the wire bonds 16 too close to the neck portion 52 is this can break the wire. The skilled worker will appreciate the work hardening increases the brittleness of the metal. The applicant's testing has found that positioning the wire engaging structure 50 so that its tip is between 1.0 mm and 1.6 mm behind the tip of the wedge (with respect to its movement from the die to the flex) and 50 microns to 600 microns below the tip of the wedge produces suitable results. In particular, the best results were achieved with the tip of the wire engaging structure 50 positioned 1.2 mm to 1.5 mm behind the tip the wedge, and 100 microns to 300 microns below the tip of the wedge. This configuration gave wire bonds 16 less than 50 microns above the nozzle array 38, each with a bond strength between 3.5 g and 5 g.

The wire engaging structure 50 is formed from a material with the surface hardness less than that of the wire. This avoids surface indentations on the wire which may later become stress concentration sites.

Adhering the Wire Bond to Reduce Loop Heights

FIG. 10 is a schematic section view of another technique used to reduce the height of the wire bond loops. An adhesive surface 62 is positioned on the LCP molding 6, between the contact pads 10 of the printhead IC 4 and the corresponding conductor 12 on the flex PCB 8. The Applicant has found that a wirebonder will usually allow the wire bond 16 to drape downwards and touch the surface between the die and PCB while the wire is bonded to the PCB conductor. Once the wire has been welded to the contact pad on the die, the bonding tool draws it towards the conductors on the printed circuit board. As it is drawn across the gap between the die and printed circuit board, the wire drapes downwardly and rests on the underlying surface. Once the bonding tool has welded the other end of the wire to the conductor, and the wire clamp immediately behind the bonding tool breaks off the feed wire by pulling until tensile failure, the residual tension in the loop cause it to bow upwards. By positioning adhesive 62 at the point of contact between the wire and the LCP 6, the wire bond 16 is unable to bow upwards to the same height.

The adhesive surface 62 may be double sided tape, an adhesive paste or resin jetted onto the LCP 6 when the die 4 and the flex 8 are fixed, or it could simply be an extension of the die attach film 58.

The invention has been described herein by way of example only. The ordinary will readily recognize many variations and modifications which do not depart from the spirit and scope of the broad inventive concept. 

1. An electronic device comprising: an integrated circuit die with a plurality of contacts pads; a printed circuit board with a plurality of conductors corresponding to each of the contact pads respectively; wire bonds electrically connecting each of the contact pads to the corresponding conductors; and, an adhesive surface positioned between the contacts pads and the corresponding conductors; wherein, the wire bonds are secured to the adhesive surface.
 2. An electronic device according to claim 1 wherein the adhesive surface is one side of a double-sided adhesive tape.
 3. An electronic device according to claim 1 wherein the integrated circuit die and the PCB are mounted to a supporting structure such that they are adjacent and spaced from each other.
 4. An electronic device according to claim 3 wherein the PCB is a flexible PCB and the supporting structure is a liquid crystal polymer (LCP) molding.
 5. An electronic device according to claim 1 wherein the integrated circuit die is mounted to the supporting structure by a die attach film, and the adhesive surface is provided by a portion of the die attach film.
 6. An electronic device according to claim 1 wherein the wire bonds are formed from lengths of wire with a gauge between 15 microns and 75 microns.
 7. An electronic device according to claim 6 wherein the gauge is about 25 microns.
 8. An electronic device according to claim 6 wherein the wire bonds attach to respective contact pads on the IC die and the wire bonds do not extend more than 150 microns above the contact pads of the IC die.
 9. An electronic device according to claim 8 wherein the line of wire bonds does not extend more than 50 microns above the contact pads of the IC die.
 10. An electronic device according to claim 1 wherein the contact pads are spaced from the corresponding conductors on the PCB by more than 1 mm.
 11. An electronic device according to claim 10 wherein the contact pads are between 2 mm and 3 mm from the corresponding conductors on the PCB.
 12. An electronic device according to claim 1 wherein the PCB is a flexible PCB mounted to a support structure together with the die such that the conductors are adjacent the contact pads on the die.
 13. An electronic device according to claim 12 wherein the support structure has a chip mounting area for supporting the die, the die having a back surface in contact with the chip mounting area and an active surface opposing the back surface, the active surface having the contact pads, and the chip mounting area being raised relative to the remainder of the support structure such that the contact pads are raised relative to the conductors.
 14. An electronic device according to claim 6 wherein the wire bonds is formed using aluminium wire.
 15. An electronic device according to claim 12 wherein the active surface has functional elements spaced less than 260 microns from the contacts pads of the die.
 16. An electronic device according to claim 15 wherein the die is an inkjet printhead IC and the functional elements are nozzles through which ink is ejected.
 17. An electronic device according to claim 16 wherein the printhead IC is configured to be mounted in a printer such that during use the nozzles are less than 100 microns from the paper path. 